help, cr125 valves

[jimyt]

i just rebuilt my brothers 98 cr125. new crank , piston, and new cylinder. i took the exhaust valve out of the old cylinder and put it in the new one. any tips on how to set the exhaust valves?
when i first got it back togeather it fired up and died right away. it did several times and i was told the exhaust valves were not working right.

[mojoe2878]

valves on a two stroke:confused:
are you talking about the reed valves or power valves? are they the thin plastic looking pieces that your talkin about?
there is no adjusting them that i know of.
when you put the new piston in did you remove (file down) a small portion of the piston at the front where it passes the reeds?these reeds arent for idling they are mainly the source of the engines power band.
does it keep running if you give it gas after it cranks or will it still die out when you give it throttle? two stroke dirt bikes arent good at idling anyway, you gotta keep them running with the throttle.

[jimyt]

mojoe ,


yes there is valves on a 2 stoke ( dirt bikes only i believe )
im not talking about reeds , ram valves , or power valve. im talking about exhaust valve. the more rpm's opens up the exhaust valves farther. i did get it running and tuned in. it idles just fine and runs good. my brother is happy with it. it just took a little while to think about how it worked. its kinda simple realy after you get it right once.:)

[Out_Sider]

dude.. you see those holes in the sleeve of that top end.. those are your valves on a 2 stroke

[jimyt]

1998 - CR125R - EXHAUST VALVE

http://www.planethondaracing.com/cgi-bin/hondaparts/results_parts_details?mv_session_id=MXmBybXr&mv_pc=107&se=pg_d_id%3d89%20and%20pg_my_id%3d3

:huh

[derekhonda]

so what opens and closes these valves, your cam? kidding, ive never heard of valves on a two-stroke.

[jimyt]

click on the link above. when it opens up look at parts number 19 and 20. its a little tuff to explain being the only time ive ever seen valves on a 2-stroke my self. part number 19 is a governor and kinda acts like a snowmobiles clutch. there is a spring on that governor that tightens up when the rpm's go up. now when that spring tightens up it turns the gear on part #20 ( rod, tie i guess its called ) just enough to pull the rod down. that rod is connected to a shaft #10 ( shaft pinion ) that goes all the way through the jug. the valves are connected to that shaft with all the other parts you see on that page. when that rod pulls down it rotates the shaft pinion in a clockwise motion about a quarter turn to open the valves up all the way.


this 2-stroke with valves is new to me too. thats why i had to ask , see if someone else new about them. i looked at my cousins 94 cr80 and it has it too , just a different set up. same with 92 kx125 and 89 yz125. what still stumbles me is it just dirtbike motors that has this and why.? i guess maybe because they dont have counter balancers. i dont know....:)

[jar944]

Exhaust valves Or power valves as most people know them have been around in motocross bikes for over 20 years.

Here is a paragraph out of Eric Gorr's book "Motocross and Off-Road Performance Handbook".
"The second-generation design features valves that control the effective stroke and the time-area of the exhaust port. These valves are fitted to the sub-exhaust ports and the main exhaust port. The main exhaust-port valves operate within close proximity to the piston to control the effective stroke of the engine. The effective stroke is defined as the distance from TDC to when the exhaust port opens. At low rpm, the engine needs a long effective stroke, which results in a high compression ratio. At high rpm, the engine needs a shorter effective stroke, longer exhaust duration, greater time-area, and a lower compression ratio. Yamaha used this system starting in 1982 on the YZ250. Honda’s HPP system is similar and was used on the 1986–91 CR250 and 1990 to current-model CR125."

EDIT.. Some atv manufacture's did include them on their 2 stroke models. Suzuki saw fit to include a powervalve on the Lt250r. Kawasaki also gave their tecate one. honda and yamaha on the other hand decided aganst it with the 250r and the banshee.

It seems to me that the manufactures are not really concerned with the performance of atv's. take a look at the past ie the 250r and the banshee both engines basically came from motorcycles. by the time the engines made it into the quad frame they had lost their powervalves and hp. 250r 25-27 hp CR250 40+ hp, Banshee 34 hp RZ350(the bike the banshee engine came from) 54 hp. It's the same today with the new four strokes. both the CRF and the YZF make more power than the 450r and the YFZ it's not quite the 20 hp of the past but it's still a lot .

[jimyt]

thanks jar. nobody else knew what i was talking about.:D
that is a good break down of it.

[dariusld]

Originally posted by jimyt
thanks jar. nobody else knew what i was talking about.:D
that is a good break down of it. nobody else:rolleyes: And not all powervalves(they have different names) work the same. The manufacturers have different ways they are actuated. Some require timing, some don't.
I don't know alot, but I don't pretend to either.

[MadMax27]

Kawasaki- KIPS Honda-HPP Suzuki- ATEV Yamaha-Powervalve

Yamaha--Originators of the exhaust valve. Yamaha designed it in the early eighties. Suzuki has a knock off. Honda is a ***** to tune. Kawasaki has the worst design from a service stand-point. The new Yamaha Powervalve is now 3 seperate valves, 2 outside and 1 centered valve. By far a superior design and performance. What a exhaust valve does: Designed to give the Engine more low end and mid range power. If your go for a bigger bore, better check the exhaust valves clearance!!!! Older versions include: Butterfly, Wedge valves. The new versions, actually driven by the crankshaft with centrifugal action. These control exhaust gas velocity, gas temp and the pressure of the compression wave. The whole point of the valve(s) is to open at low RPM to allow the motor, which is a long effective stroke, which makes high compression, while at a higher rpm, the engine produces a shorter effective stroke, and longer exhuast duration (time). Thus the valve controls your Power ina BIG way. If yer bike sucks at low rpm, but pulls hard on the top, look at yer valves, they may be stuck with mung. Sorry if this is repetative, I didnt read the link.

[AdvantageCycle]

I have got to jump in here.........some posts are joking about the cam in a 2 stroke.............there are 2 stokes that used cams and valves just like a 4 stroke. Just never made it to the powersports industry...........Detroit Diesel used 2 stoke engines in lots of applications.